This disclosure is directed to a high voltage, low impedance output amplifier to be used in a down hole logging tool. It is particularly intended for use in a logging tool which is normally operated on the end of a supportive electrical cable of perhaps 25,000 feet in length. The tool is normally lowered into a bore hole and is exposed to ever increasing temperatures as it traverses the bore hole. The depths, encountered with current drilling technology indicate that the device must operate at down hole temperatures of perhaps 275.degree. C., a temperature routinely encountered in deep wells.
This amplifier is an output amplifier. It forms an output signal which is coupled to the cable. The output signal must be fairly large because the cable will often be 25,000 feet in length. The cable is always longer than the hole which is being tested inasmuch as a portion of the cable normally remains on the spool from which it is supplied. Signal degredation must be avoided in the transfer of this analog signal to the surface. A clean signal free of distortion must be obtained to enable data processing equipment to convert the signal after the signal has been received at the surface via transmission by the logging cable.
This disclosure sets forth an unobvious arrangement of VMOS transistors. VMOS transistors are desirable in certain regards. While a VMOS transistor has several advantages, one unavoidable detriment of the VMOS transistor is the fact that it has a temperature dependent bias drift. Normally, a VMOS transistor is voltage biased. The bias threshold is typically dependent to the extent of about 7 millivolts per degree centigrade temperature change. On encountering a 200.degree. temperature change, bias threshold voltage will change about 1.400 volts. A fluctuation of this size completely obviates successful operation of the transistor.
It has been discovered that an output amplifier stage of VMOS transistors can be biased by the use of a third VMOS transistor to form a bias voltage. The bias voltage is inverted to obtain the proper polarity of the bias drift and is then applied to the output transistor stage. Bias tracking is assured by placing the third VMOS on a common heat sink with the output transistor controlling output current.
There are many methods of arranging output transistors in an amplifier configuration. It is desirable that the output transistor stage form fairly large voltage swings working into a low impedance cable. To achieve this kind of driver amplifier which is temperature stable over such a wide range is highly desirable.